Beginning in April of 2009, Google will start tracking user activity via a cookie. This tracking cookie will then determine what ads might be most appropriate for you, based on your interests. People who frequently visit ESPN.com, for example, might see sports-related ads. The logic is that you will be more interested in these ads than you might be in ads that are simply related to the content of the blog.
For more information, or to opt out, go to Google Privacy Center. There is a blue "opt out" button in the upper right corner.
If you opt out, you will continue to see ads, but they will be related to the content of the blog - something that does not require a cookie.
this video cheered me up for the whole day